Method of separation of magnetic particles of high retentivity from those of low retentivity



May 17, 1955 v. c. WILSON 2,708,518 METHOD OF SEPARATION OF MAGNETIC PARTICLES OF HIGH RETENTIVITY FROM THOSE OF LOW RETENTIVITY Filed June 29, 1950 FigJ.

L aw RETEA/r/ V/ 7') PARTICLES Maw MAGNET/c PLATE Fig.2.

Inventor: Voh'wey C. Wilson,

His Attorney.

United States Patent METHOD OF SEPARATION OF MAGNETIC PAR- TICLES OF HIGH RETENTIVITY FROM THOSE OF LOW RETENTIVITY Volney C. Wilson, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 29, 1950, Serial No. 171,164

2 Claims. (Cl. 209-214) This invention relates to a method of separation of particles or powders of magnetic materials and more specifically to a method forthe separation of high magnetic retentivity particles from low magnetic retentivity particles.

It has recently become evident that permanent magnets having superior magnetic properties, including higher magnetic retentivity, may be produced by powder metallurgy from very fine magnetic powders. The desirable magnetic properties of these magnets can be materially improved if some assurance can be had that the'powders which are employed in the production of these magnets contain a very high percentage of permanent magnetic material. Therefore, it is important to have a simple, convenient and reliable method for separating high magnetic retentivity particles from low magnetic retentivity particles in order that the high retentivity particles may be used in the production of these magnets. Other instances will of course be found when separation of high retentivity from low retentivity magnetic particles is highly desirable.

It is therefore an object of this invention to provide an improved method for separating permanent or high retentivity magnetic particles from soft iron or low retentivity magnetic particles.

This invention therefore consists essentially in a method for the separation of high retentivity from low retentivity magnetic particles which includes the steps of subjecting the magnetized mixture of particles to an alternating magnetic field through a support plate of non-magnetic material to cause migration of the high retentivity particles towards the periphery of the field while retaining the low retentivity magnetic particles near the center of the field.

For a more complete understanding of this invention, reference should be had to the following specification and the accompanying drawing in which Fig. l is a top plan view, and Fig. 2 is a sectional side view of an apparatus which may be used in carrying out the method of this invention.

Referring more particularly to Fig. l of the drawing, there is shown a plate 1 which is of a suitable non-magnetic material of a proper thickness to give a certain degree of rigidity and mechanical strength. A glass plate of one-eighth of an inch in thickness was found to be satisfactory. Beneath the plate 1 there is an electrical solenoid 2 having a center opening at 3, which is approximately four inches in diameter. A suitable alternating current voltage may be supplied at the solenoid winding leads 4.

In the contemplation of this drawing, the observer is looking vertically downwardly at the apparatus shown. The apparatus is positioned preferably on a work table with the solenoid having its axis vertical and the plate 1 is placed upon the upper end of the solenoid. A small amount of the powder consisting of magnetic particles to be separated is placed at the center of the plate at approximately the position designated at 5. The mag- 2,708,518 Patented May 17, 1955 netic particles are preferably previously magnetized in a strong D. C. magnetic field. After placing the particles at the center of the plate, a 60 cycle alternating magnetic field of approximately oersteds peak is applied to the particles by connecting an appropriate voltage to the coil winding leads 4. The permanently magnetized or high retentivity magnetic particles will then rapidly move radially outwardly to form a ring, as indicated at 6, at a radius slightly larger than the radius of center opening 3 of the solenoid 2. The low retentivity magnetic particles, however, remain near the center of the solenoid where they form clusters as shown at 5. The separative action may be made more complete if the initial clusters at the center of the solenoid are mechanically agitated and broken up by a non-magnetic instrument (not shown) so that high retentivity particles which may be mechanically entrapped by the low retentivity particles may escape to join the separated ring of high retentivity particles at 6. A suitable non-magnetic instrument for this purpose is an ordinary brush, such as a paint brush or a dusting brush. When the separation is complete, as it will be within a minute or two, depending upon how large a sample of powder is used, the outer ring of high retentivity particles at 6 may be brushed radially outwardly from the edge of the plate into a suitable container leaving only the low retentivity particles at the center of the plate which may be put into a second container.

Fig. 2, which is a view through the section 2-2 of Fig. 1, further illustrates the positions and configurations of the apparatus components.

The frequency of the alternating magnetic field is not critical. However, the frequencies under 1000 cycles were found to give better results, and the lowest frequencies, in the order of 60 cycles, appeared to work best of all. Particle sizes in the powders to be separated are apparently also uncritical. Particle sizes ranging from one-half micron to one hundred microns were used satisfactorily in tests of this method. However, smaller particles appeared to respond more readily to the higher frequencies, while all of the particles appeared to respond to the lower frequencies. Magnetic particles may therefore be sorted according to sizes by employing the method of this invention in separate steps of decreasing magnetic frequencies.

The particles must be highly magnetized before they are placed in the alternating magnetic field, and if an alternating magnetic field of much higher intensity than the 125 oersteds peak specified above is initially applied, a demagnetization of the particles may occur and the separation may be imperfect.

One theory to explain the operation of the method of this invention is that the low retentivity magnetic particles have their magnetic polarities easily and quickly reversed in the rapidly reversing alternating magnetic field and are quickly attracted to the area of highest magnetic density above the center of the coil. The high retentivity particles, retaining their permanent magnetism, are alternately attracted and repelled in the alternating magnetic field, the repulsion phase of these particles causing them to move upward and outward, following the outwardly slanting lines of magnetic force and then falling straight down under gravity force to gradually move toward the periphery of the field generated by the solenoid. These particles, being polarized, could, of course, swing around for attraction instead of repulsion, but the mechanical inertia and the friction on adjacent particles undoubtedly inhibit this action. The outwardly fanning lines of magnetic force obtainable with such an axially short and diametrically large toroidal coil as that disclosed are thought to be important in order to get a maximum horizontal component of travel of the permanent magnet particles on each repulsion cycle. Slanting What I claim as new and. desire to secure by Letters Patent of the United States is:

1. A. method for separation of high retentivity from low retentivity magnetic particles comprising the. steps of strongly magnetizing said particles, placing, the rnagnetized particles on a horizontal support within an alternat-- ing magnetic field having, non-vertical components and. an area substantially larger than. the area covered by said particles, mechanically agitating the magnetized particles within the area of their initial. placement until. separation in space between the low retentivity and high. retentivity magnetic particles is accomplished by movement of the high retentivity particles toward the periphery of the magnetic field.

2. The method of separating high retentivity from low retentivity magnetic particles comprising the steps of magnetizing the particles by subjecting them to a strong unidirectional magnetic field above 125 oersteds, producing an alternating toroidal magnetic field no greater than 125 oersteds peak amplitude, supporting, the mag.-

netized particles in an area substantially at the center 4 of the toroidal magnetic field, and agitating the particles in such central area until separation in space between the low retentivity and high retentivity magnetic particles is accomplished by the movement of the magnetized high retentivity particles toward the periphery of the toroidal magnetic field.

References Cited in the file of this patent UNITED STATES PATENTS.

653,345 Gates July 10, 1900 940,282 Rogers Nov. 16, 1909 1,417,189 McCarthy May 23, 1922 2,230,344 Bair Feb. 4, 1941 2,429,436 Walker Oct. 21, 1947 2,470,889 Drescher May 24, 1949 2,487,272 Price Nov. 8, 1949 FOREIGN PATENTS 224,924 Great Britain. Nov. 17, 1924 OTHER REFERENCES Three Phase A. C. Can Improve Fine-Size Magnetic- Separation. Engineering and Mining, Journal; volume 152, Issue 10, pp. 82, 83, 118. October 1951..

R. I.. 4057, U. S. Bureau of Mines, by Buehl, Shauband Riott. March 1947. Pages 18, 19.

R. I.. 3229-, U. S. Bureau of Mines AlternatingCurrent Ma-gnetic- Separation of. Iron Ores, by Davis. May 1934. Pages 35-37. 

1. A METHOD FOR SEPARATION OF HIGH RETENTIVITY FROM LOW RETENTIVITY MAGNETIC PARTICLES COMPRISING THE STEPS OF STRONGLY MAGNETIZING SAID PARTICLES, PLACING THE MAGNETIZED PARTICLES ON A HORIZONTAL SUPPORT WITHIN AN ALTERNATING MAGNETIC FIELD HAVING NON-VERTICAL COMPONENTS AND AN AREA SUBSTANTIALLY LARGER THAN THE AREA COVERED BY SAID PARTICLES, MECHANICALLY AGITATING THE MAGENTIZED PARTICLES WITHIN THE AREA OF THEIR INITIAL PLACEMENT UNTIL SEPARATION IN SPACE BETWEEN THE LOW RETENTIVITY AND HIGH RETENTIVITY MAGNETIC PARTICLES IS ACCOMPLISHED BY MOVEMENT OF THE HIGH RETENTIVITY PARTICLES TOWARD THE PERIPHERY OF THE MAGNETIC FIELD. 